Researcher dives deep into the hidden world of stainless steel

Luqing Cui is a young researcher who is passionate about materials science, more specifically the mechanical properties of metal alloys. His work is dedicated to understanding stainless steel at a submicron scale where so called cell structures are formed.

Teiksma Buseva

Luqing Cui was appointed to a postdoctoral position in the Division of Engineering Materials, and joined the team in November 2019. Two years of intensive work has resulted in eight peer-reviewed articles. This is an impressive accomplishment for a young researcher at the beginning of his career.

Additive manufacturing (AM), with its unique advantages of design freedom and rapid manufacturing capability, has gained huge scientific interest. Researchers all over the world are examining the limits of the technology, and finding new approaches to fill knowledge gaps. They are seeking to fully understand the materials used in additive manufacturing.

Cellular structure and exposure to load

Luqing Cui studies the fatigue properties of AM alloys. Fatigue is a process that leads to mechanical failure, and it results from the application of repeated cyclic stresses. In other words he studies how the unique microstructure in the material, especially the solidification of cellular structure, is affected by the continual and repeated loading.

“Many people study additive manufacturing. My research is mainly about the relationship between microstructure and the mechanical properties of additive manufacturing materials. My focus is entirely on deformation and failure mechanisms.”

To be able to introduce AM for critical and highly loaded metallic products, we need a deeper understanding of the processes and materials.

“There are many studies on cellular structure and tensile properties, but only a few have looked at the fatigue properties. These are more complex than tensile properties, and are usually encountered when studying engineering materials in practical use”, says Luqing Cui.

The research group at Linköping University has collaborated with industry for many years to study the performance of the AM materials used in demanding applications in the aerospace and gas turbine industries. Even small changes in the proportions of the elements in an alloy composition can lead to significant changes in behaviour and performance.

“We need to know if a material is reliable or not, and this means that we must do tests. In our lab we look at fatigue processes at high temperatures. We perform creep tests at 1000 degrees, and high-pressure tests at 300 megapascal. We look at how much time it takes to reach failure, and how the microstructure evolves during the deformation process. This information also helps us suggest how the microstructure can be adjusted, and mechanical properties improved, for alloys in the future.”

Work hard, dig deep and you will discover unexplored areas.
Luqing Cui , researcher at the division of Engineering Materials

Dislocations are line defects that exist in metals and play a crucial role in determining the strength and plasticity of the material. Luqing Cui and his colleagues have seen that the existence of cellular structures promotes the activation of planar slip, delays strain localisation, and ultimately enhances the fatigue performance of additively manufactured stainless steel.

“We suggested a new approach to determine the density of geometrically necessary dislocations and statistically stored dislocations. Our approach is based on an indentation size effect and strengthening theories. It is much simpler than the conventional methods used, TEM and EBSD, and it is time-efficient and low-cost. Therefore, it has a great application potential in the field of mechanical performance analysis and microstructure tailoring of metallic materials.”

Microstructures prior to deformations (a-f) and post-fatigued (g-h)

Differences in academic and social culture 

Luqing Cui is very grateful for the opportunity to do research in Sweden. He found the research environment here to be very different from that in China. 

“In China, we must do other things than just research. We have to help our supervisor, for example, with administration or financial reports, or download and collate the required literature. We also have to clean the labs, and carry out projects for external companies. In China much of our time is dedicated to other things than research. Here I can spend 100% of my time on research.” 

Luqing Cui thoroughly enjoys being in Sweden, where he admires the green nature with its abundance of trees. The journey has, however, not been without some challenges. The time difference between China and Sweden has made it harder to communicate with his family. The dinner traditions and the availability of groceries have also been challenges. 
“Swedish food is very different from Chinese food. Vegetables here are very expensive and the variety of vegetables limited, while meat is very cheap compared to China. I also experience that there are differences when it comes to dinner traditions. In China, dinner is often served much warmer”, laughs Luqing.

Due to the pandemic, Luqing has not been able to visit his family as often as he wished. He has been back to China only once since 2019. He plans to return to China in December 2021, where he will be reunited with his wife and two-year-old daughter. 

 

Supervisor about the work of Luqing Show/Hide content

Luqing has been incredibly ambitious and very productive. In our research, we often focus on the relationship between properties and structure of metallic materials. Luqing has not only looked at what the relationship look like, but has really tried to get to the bottom of trying to explain why the relationship look the way they do. To do this, one must take advantage of very many different methods, both experimental and theoretical. Luqing has a very large width and depth and is therefore able to solve a very advanced puzzle of clues.
Johan Moverare, Professor in Engineering Materials

Vocabulary Show/Hide content

Alloy

Mechanical properties

Tensile properties

Fatigue properties

Solidification cellular structures

Deformation

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